Among various compound semiconductors, II-VI compound semiconductor has a particularly wide band gap and is capable of emitting typically yellow, green and blue lights. Recent efforts are thus directed to development of photoelectric conversion device using a II-VI compound semiconductor crystal as a base material and having a high efficiency and long life time.
While it is necessary that, in order to fabricate photoelectric conversion device with a high efficiency, semiconductor can arbitrarily be controlled into both conductivity types of n-type and p-type, it is known to be difficult to control the conductivity type of II-VI compound semiconductor in general. For example, it is relatively easy to control ZnSe-base compound semiconductor so as to have n-type conductivity and low resistance, but it is difficult to control it so as to have p-type conductivity. On the contrary, ZnTe-base compound semiconductor can be controlled to p-type in a relatively easy manner, but is difficult to be controlled to n-type.
It is to be noted now that ZnSe-base compound semiconductor refers to semiconductors including any compound semiconductors which can substantially establish lattice matching with ZnSe, and ZnTe-base compound semiconductor similarly refers to semiconductors including any compound semiconductors which can substantially establish lattice matching with ZnTe.
In general, ZnSe-base compound semiconductor crystal used for photoelectric conversion device contains a p-type ZnSe layer formed by the molecular beam epitaxy in which highly reactive activated nitrogen excited in plasma is irradiated into ZnSe at a high dose, where thus-obtained p-type ZnSe layer is used as a contact layer for ensuring contact with an electrode.
It has, however, been difficult for the ZnSe-base compound semiconductor to achieve a high concentration of p-type carrier, and as a consequence it has been impossible to sufficiently lower contact resistance between a p-type electrode and the p-type ZnSe contact layer. For this reason, photoelectric conversion deivces using the ZnSe-base compound semiconductor crystal having a p-n junction therein have been suffering from problems of higher operation voltage, larger power consumption, and accelerated deterioration of the devices due to generated heat.
There was thus proposed a photoelectric conversion device having a ZnTe layer, which could accept a larger dose of p-type impurity, formed on the p-n junction of ZnSe-base compound semiconductor, making use of the ZnTe layer as a contact layer with a p-type electrode. The device was, however, unsuccessful in lowering of the operation voltage since the ZnTe layer directly stacked on the ZnSe layer undesirably raised electric resistance due to a large energy gap at the interface therebetween.
There were also developed a photoelectric conversion device using a crystal having a layer structure which comprised a ZnSe layer, a ZnSeTe composition-graded layer formed thereon and having a compositional ratio of Se and Te gradually varied, and a p-type ZnTe contact layer formed further thereon; and a photoelectric conversion deivce using a crystal having a layer structure which comprised a ZnSe layer, a ZnSe/ZnTe superlattice layer formed thereon, and a p-type ZnTe contact layer formed further thereon, which were aimed at lowering the operation voltage.
It is, however, difficult to ensure an excellent crystallinity for the above-described crystals having the layer structures containing the ZnSeTe composition-graded layer or the ZnSe/ZnTe superlattice layer due to a large difference in the lattice constant between ZnTe and ZnSe, which disadvantageously exerts adverse effects on the characteristics of the photoelectric conversion deivces. Although the photoelectric conversion devices using ZnSe-base compound semiconductor crystals having the above-described layer structures have already been put into practical use, they are failing in sufficiently lowering the operation voltage, and they are far from being fully improved in the power consumption and device deterioration.
On the other hand, not so much efforts have been directed to development of the photoelectric conversion deivces using ZnTe-base compound semiconductor due to difficulty in growing ZnTe single crystals. The present inventors have, however, recently succeeded in growing p-type ZnTe single crystals, which has broken a way to stable procurement of the p-type ZnTe single crystal substrate. This has promoted a wide movement towards development of the photoelectric conversion devices using the ZnTe-based compound semiconductor.
The ZnTe compound semiconductor crystal used as a base material for the photoelectric conversion device generally has any of layer structures described below.
One example of the layer structure is obtained by placing a group XIII (IIIB) element on the surface of the p-type ZnTe substrate, and allowing the group XIII (IIIB) element to diffuse by annealing into the substrate as an n-type impurity so as to form a ZnTe layer.
Another example of the layer structure is obtained by forming an n-type ZnTe thin film by the epitaxial growth method on the p-type ZnTe substrate.
Still another example of the layer structure is obtained by sequentially forming on the p-type ZnTe single-crystalline substrate a p-type ZnTe buffer layer, a p-type clad layer comprising a quaternary mixed crystal containing Mg, Cd, Se or the like, which is typified by MgZnSeTe and CdZnSeTe, a ZnTe active layer, and an n-type clad layer comprising a quaternary mixed crystal.
It has, however, been difficult for the ZnTe-base compound semiconductor to achieve a high concentration of n-type carrier, and as a consequence it has been impossible to sufficiently lower contact resistance between an n-type electrode and the n-type ZnTe layer. For this reason, photoelectric conversion deivces using the ZnTe-base compound semiconductor crystal have been suffering from problems of higher operation voltage, larger power consumption, and accelerated deterioration of the devices due to generated heat.
That is, the carrier concentration in the n-type semiconductor layers (n-type ZnTe layer, n-type clad layer comprising a quaternary mixed crystal) in the above-described layer structure can be raised only to as high as 1017/cm−3, which is not a level of carrier concentration capable of establishing ohmic contact, and it is thus difficult to lower the contact resistance. Therefore the photoelectric conversion devices using the ZnTe-base compound semiconductor crystal having a p-n junction have not reached a level suitable for the practical use, and there are no reported cases succeeded in lowering of the operation voltage.
The present invention is therefore completed so as to solve the foregoing problems, and is to provide a II-VI compound semiconductor crystal having a contact layer which can be controlled to a desired conductivity type, and a photoelectric conversion device using thereof as a base material.